1. Field of the Invention
The present invention relates to a photoconductor for electrophotography, and more specifically relates to a photoconductor having a thick film photoconductive layer and which provides excellent electrical characteristics and wear resistance characteristics.
2. Description of the Related Art
In recent years, there has been various research done relating to photoconductors, and photoconductors of the function-separated type have been developed wherein the photoconductive functions of charge-generating function and charge-transporting function are provided by separate materials. Typically, the photosensitive layer of photoconductors of the laminated, function-separated type comprise a laminate structure of a charge-generating layer including a charge-generating material, and a charge-transporting layer including a charge-transporting material and binding resin, whereas the photosensitive layer of photoconductors of the dispersed, function-separated type comprise a layer having a charge-generating material and a charge-transporting material dispersed within a binding resin.
Laminate type photoconductors of the aforesaid function-separated type allow a broad range of selectable materials, and can produce high performance photoconductors by allowing the inclusion of ideal materials for electrophotographic characteristics such as charging characteristics, sensitivity, residual electric potential, repetition characteristics and the like. Inexpensive photoconductors can be provided by application processes in production so as to make extremely high production possible, and photosensitive wavelength range can be freely controlled by suitably selecting charge-generating materials.
The aforesaid photoconductors, however, generally have reduced mechanical strength, and inferior wear resistance, and wearing of the photoconductor layer caused by loads occurring within the device under practical conditions such as friction with paper, friction with a cleaning member and the like reduces layer thickness. The amount of reduction of the layer due to friction differs depending on materials and processing, but a thickness reduction of about 0.2.sup..about. 1 .mu.m after processing 10,000 sheets is typical. Reduction of layer thickness leads to a reduction in charging characteristics. When the range of permitted charging reduction is exceeded, the service life of the photoconductor is approached and, as a result, printing resistance deteriorates.
Methods for improving the sensitivity and wear resistance of photoconductors have included, for example, techniques for making the thickness of the photoconductor layer thicker than in the past.
When the thickness of a photoconductor layer is simply increased markedly, the service life of the photoconductor is certainly lengthened, but various disadvantages arise inasmuch as film thickness irregularity, and insufficient cleaning of the photoconductive member surface also occur. Furthermore, when processing of several hundred copy sheets has been accomplished, uneven density occurs in images, leading to unsharp images.
The previously mentioned disadvantages are largely the causes of damage, wear, and deterioration due to mechanical or physical external forces during printing resistance and particularly application state of charge-transporting layers, e.g., degree of applicability, of photoconductor layers and laminate layers. These factors are greatly dependent on the characteristics of the binding resin used in forming the photoconductor layer.
General immersion application methods may be used as photoconductive layer forming methods for photoconductors. Immersion application methods comprise immersing a substrate in a vessel filled with an application fluid, so as to form an application layer on the surface of the substrate by lifting the immersed substrate vertically at constant speed. Although immersion application methods allow relatively easy formation of uniform thin layers, when used for the formation of thick layers, the resin type and characteristics can cause variation in the application state of the layer and the characteristics of the photoconductor.
In general, when a high molecular weight resin is used as the binding resin of a photoconductor, the surface hardness of the photoconductor is increased, thereby providing excellent wear resistance, but conversely making it difficult to remove residual toner adhering to the surface of the photoconductor, such that image noise is produced due to a "filming" phenomenon. On the other hand, when low molecular weight resin is used as the binding resin, the aforesaid "filming" can be prevented, but the hardness of the resin is reduced, which tends to adversely affect wear resistance and make the photoconductor more susceptible to deterioration due to ozone and the like. Forming a thick photoconductor layer is difficult because when the viscosity of the binding resin is too great, a uniform application of the photoconductor layer cannot be achieved, and when said viscosity is too low, liquid runs occur which prevent uniform application of the layer.
When a resin having one type of molecular weight distribution is used as a binding resin, dispersions in the molecular weight distribution may occur by the manufacturing lot, thereby making it difficult to regulate the viscosity of the application liquid. This is disadvantageous from the perspective of manufacturing stability inasmuch as the strength of the application layer is not uniform.